This invention relates to catalysts for decomposing ammonia (NH.sub.3) to nitrogen gas (N.sub.2) and water (H.sub.2 O) by oxidation for the treatment of ammonia contained in various exhaust gases.
Ammonia is a harmful substance with a noxious odor, and legal regulations are imposed on the quantity of ammonia to be released into the atmosphere. In recent years, research efforts have been directed to the denitration process which uses NH.sub.3 as a reducing agent for the selective catalytic reduction of nitrogen oxides (NO.sub.x). With this process, nitrogen oxides are reduced to harmless N.sub.2 with NH.sub.3 at a temperature of 300.degree. to 400.degree. C. in the presence of a catalyst. To effect the denitration reaction rapidly, steadily and continuously, NH.sub.3 is usually fed at a rate in excess of the stoichiometric amount, consequently permitting an excess of NH.sub.3 to be released into the atmosphere along with the treated gas. In the case where the denitrating apparatus is equipped with a wet-type desulfurizing unit downstream therefrom, NH.sub.3 is likely to dissolve into the desulfurizing absorption liquid and will be discharged as contained in the effluent. Moreover NH.sub.3 will react with the untreated sulfur oxides (SO.sub.x) passing through the denitrating apparatus, forming (NH.sub.4).sub.2 SO.sub.3 , (NH.sub.4).sub.2 SO.sub.4, NH.sub.4 HSO.sub.3, NH.sub.4 HSO.sub.4 and like salts which would clog up the heat exchanger and other devices disposed downstream from the denitrating apparatus.
In order to eliminate these problems, the quantity of NH.sub.3 flowing out from the denitrating apparatus must be minimized to the greatest possible extent. For this purpose, we have carried out research on the use of NH.sub.3 decomposing catalyst at a location downstream from the reaction layer within the denitrating reactor as the most feasible method. Such catalysts for decomposing NH.sub.3 by oxidation must have the following characteristics.
(1) High activity to oxidize and decompose NH.sub.3 which can be sustained with stability within the range of reaction temperatures for denitration.
(2) Low activity to convert NH.sub.3 to NO.sub.x.
(3) Freedom from poisoning by SO.sub.x contained in exhaust gases.
(4) Reduced activity to oxidize SO.sub.2, contained in emissions, to SO.sub.3. (SO.sub.3, if formed, would raise the dew point of exhaust gages, create corrosion in devices and cause air pollution, hence objectionable.)
(5) Usability in the form of a packed layer with a reduced pressure loss.
NH.sub.3 -containing exhaust gases further include those resulting from the combustion or thermal decomposition of nitrogen-containing substances such as industrial wastes and those released from flues and other devices in which NH.sub.3 is intentionally added to the exhaust gas to inhibit the corrosion of the device. Gases discharged from electric dust collectors similarly contain NH.sub.3 which is added thereto to ensure improved dust removal efficiencies. These gases also require the removal of NH.sub.3 before disposal.
The catalysts for decomposing NH.sub.3 by oxidation heretofore proposed include those comprising an oxide of Cr, Mn or like base metal. However, SO.sub.x, if contained in the exhaust gas, would sulfatize the oxide, rendering the catalyst less active and unusable. It is also noted that catalysts conventionally used are generally in the form of cylindrical or spherical grains, but the catalysts of such shape are unfit for the treatment of quantities of gases because of the great pressure losses involved.